I feel like some of y'all are blowing the kiss from last chapter out of proportion. It was one kiss, not directly on the lips, and ended with Yukie saying that was enough. Not only was Manaka there to see all of the context around it, but Shiori even acknowledged to Manaka that she was giving up and implied pretty directly to her that Manaka was the one Yukie liked. Additionally, they're not even actually dating to begin with? Manaka can certainly feel bad about the situation, but the only one who has any real reason to feel angry is Yukie, from how strongly Shirori came into her, and we already know that Yukie is way too nice for that.